Exacto knife blade is a little small, and gets dull fast cutting through both the paper and the foam inside. A snap knife, the kind with extendable blade that you can snap off sections might work better. Use a pair of pliers to snap of the end pieces of the knife as it gets dull. That or a razor blade knife might be better. The hot wire cutters are great for just foam, but would do nothing for the cardboard on both sides.